1. Field of the Invention
This invention relates to a method for the separation of valuable components from a mixture of aromatic compounds and more particularly to a novel process effecting substantially complete separation of meta-xylene and ortho-xylene from a mixture containing these components together with ethyl benzene and para-xylene in a rapid and economic manner.
2. Description of the Prior Art
Aromatic compounds and particularly para-xylene, meta-xylene, orthoxylene and ethyl benzene are well known as very useful materials in the chemical industry but are generally found only in admixture with each other. For example, they are found in substantial quantities in coke oven light oil, in reformed petroleum naphthas and in "pyrolysis gasoline" from steam cracking of hydrocarbons to make olefins. Over the years many processes have been devised for separation of the several components to recover the desired components of satisfactory purity. Those techniques are in use on a large scale. Present practices employ distillation, selective sorption or fractional crystallization or combinations of those unit processes. Distillation is difficult and very expensive because of the very close boiling points of the components. Among the sorption processes, proposals have been offered for chromatography and sorption/desorption processes.
The use of zeolite ZSM-5 for separation of C.sub.8 aromatics is disclosed in certain prior patents assigned to the assignee of this application. See U.S. Pat. Nos. 3,653,184; 3,656,278; 3,698,157; 3,724,170; 3,729,523; 3,760,024 and 3,770,841. A system for achieving continuity of flow by parallel systems is shown in copending application Ser. No. 310,054, filed Apr. 28, 1972. In the processes of these prior patents and application the adsorption capabilities of the zeolite are utilized to effect production chromatography or adsorption and then desorption of the components of the aromatic mixture. Since the filing of the applications of reference, however, it has been realized that the time required to effect adsorption of each component followed by desorption of each component is expensive in both capital cost and operating cost. A matter of primary importance in this regard is that desorption is much more time consuming than adsorption, all as developed more fully below. Accordingly, work has continued in this area in an effort to uncover procedures by which the adsorption characteristics of ZSM-5 zeolites can be utilized to best advantage while obtaining a rapid separation of the desirable components. It is found that the apparatus employed in the chromatographic process of U.S. Pat. No. 3,724,170 is susceptible of modification to a sorption/desorption mode which provides an effective method of series adsorption and parallel desorption for rapid separation and recovery of the C.sub.8 aromatic components.
In the prior patents held by the assignee of this application and in the general prior art relating to a separation of C.sub.8 aromatics by selective sorption, two distinct modes of operation need to be distinguished by their process characteristics, their advantages and their disadvantages.